1. Field of the Invention
The present invention relates to an electric power steering apparatus configured to transmit a driving force produced by an electric motor, to a steering mechanism for performing steering assist.
2. Description of Related Art
Electric power steering apparatuses are configured such that driving forces produced by electric motors are transmitted to steering mechanisms to perform steering assist.
In a conventional electric power steering apparatus, assist characteristics for determining the relationship between a steering torque applied to a steering wheel and a reference assist torque are determined beforehand. The assist characteristics are stored as an assist map in a memory. A reference assist torque corresponding to the steering torque detected by a torque sensor is read out of the assist map. The driving of the electric motor is controlled so as to produce a driving force corresponding to the read reference assist torque.
In a control system of this electric power steering apparatus, an output signal of the torque sensor is subjected to phase compensation, thereby enhancing the responsiveness at the time of forward stroke steering. The forward stroke steering is an operation to rotate the steering wheel in a direction away from a steering angle midpoint. If a gain in frequency response characteristics in the phase compensation is set to a large value, the driving force can be produced from the electric motor with high responsiveness to the operation of the steering wheel at the time of the forward stroke steering.
However, the enhanced responsiveness for the forward stroke steering may cause a drawback for the return stroke steering, which is an operation to rotate the steering wheel in a direction toward the steering angle midpoint. Specifically, when the driver releases his or her hands from the steering wheel during return stroke steering, the steering torque is rapidly reduced. In response, a steering assist force exerted in a forward stroke steering direction is rapidly reduced. As a result, return of the steering wheel to the steering angle midpoint becomes rapid, thereby degrading the convergence of the steering wheel.
This problem can be solved by performing the phase compensation such that the gain is reduced in a high frequency band in the frequency response characteristics at the time of the return stroke steering. If the gain is reduced in the high frequency band, however, the steering assist force at the start of the return stroke steering becomes insufficient. Therefore, there arises another problem that a driver may have a feeling that the steering wheel is caught or entrapped.